A communication system is comprised, at a minimum, of a transmitter and a receiver interconnected by a communication channel. A communication signal is transmitted by the transmitter upon the transmission channel to be received by the receiver. A radio communication system is a communication system in which the transmission channel comprises a radio frequency channel defined by a range of frequencies of the electromagnetic frequency spectrum. A transmitter operative in a radio communication system must convert the communication signal into a form suitable for transmission upon the radio-frequency channel.
Conversion of the communication signal into a form suitable for transmission upon the radio-frequency channel is effectuated by a process referred to as modulation. In such a process, the communication signal is impressed upon an electromagnetic wave. The electromagnetic wave is commonly referred to as a "carrier signal." The resultant signal, once modulated by the communication signal, is commonly referred to as a modulated carrier signal. The transmitter includes circuitry operative to perform such a modulation process.
Because the modulated carrier signal may be transmitted through free space over large distances, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
The receiver of the radio communication system which receives the modulated carrier signal contains circuitry analogous to, but operative in a manner reverse with that of, the circuitry of the transmitter and is operative to perform a process referred to as demodulation.
Numerous modulated carrier signals may be simultaneously transmitted upon differing radio frequency channels of the electromagnetic frequency spectrum. Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands, and have regulated transmission of the modulated carrier signals upon various ones of the frequency bands. (Frequency bands are further divided into channels, and such channels form the radio-frequency channels of a radio communication system.)
A two-way radio communication system is a radio communication system, similar to the radio communication system above-described, but which permits both transmission and reception of a modulated carrier signal from a location and reception at such location of a modulated carrier signal. Each location of such a two-way radio communication system contains both a transmitter and a receiver. The transmitter and the receiver positioned at a single location typically comprise a unit referred to as a radio transceiver, or more simply, a transceiver.
A two-way, radio communication system which permits alternate transmission and reception of modulated carrier signals is referred to as a simplex system. A two-way radio communication system which permits simultaneous transmission and reception of communication signals is referred to as a duplex system.
A cellular communication system is one type of two-way radio communication system in which communication is permitted with a radio transceiver positioned at any location within a geographic area encompassed by the cellular communication system.
A cellular communication system is created by positioning a plurality of fixed-site radio transceivers, referred to as base stations or base sites, at spaced-apart locations throughout a geographic area. The base stations are connected to a conventional, wireline telephonic network. Associated with each base station of the plurality of base stations is a portion of the geographic area encompassed by the cellular communication system. Such portions are referred to as cells. Each of the plurality of cells is defined by one of the base stations of the plurality of base stations, and the plurality of cells together define the coverage area of the cellular communication system.
A radio transceiver, referred to in a cellular communication system as a cellular radiotelephone or, more simply, a cellular phone, positioned at any location within the coverage area of the cellular communication system, is able to communicate with a user of the conventional, wireline, telephonic network by way of a base station. Modulated carrier signals generated by the radiotelephone are transmitted to a base station, and modulated carrier signals generated by the base station are transmitted to the radiotelephone, thereby to effectuate two-way communication therebetween. (A signal received by a base station is then transmitted to a desired location of a conventional, wireline network by conventional telephony techniques. And, signals generated at a location of the wireline network are transmitted to a base station by conventional telephony techniques, thereafter to be transmitted to the radiotelephone by the base station.)
Increased usage of cellular communication systems has resulted, in some instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communications. By more efficiently utilizing the frequency band allocated for radiotelephone communication, the transmission capacity of an existing, cellular communication system may be increased.
The transmission capacity of the cellular communication system may be increased by minimizing the modulation spectrum of the modulated signal transmitted by a transmitter to permit thereby a greater number of modulated signals to be transmitted simultaneously. Additionally, by minimizing the amount of time required to transmit a modulated signal, a greater number of modulated signals may be sequentially transmitted.
By converting a communication signal into discrete form prior to transmission thereof, thereby to form a digital code, the resultant modulated signal is typically of a smaller modulation spectrum than a corresponding modulated signal comprised of a communication signal that has not been converted into discrete form. Additionally, when the communication signal is converted into discrete form prior to modulation thereof, the resultant, modulated signal may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single transmission channel.
A transmitter which converts the communication signal into discrete form converts the communication signal into a digital code which is modulated and then transmitted upon the communication channel.
While, ideally, the signal received by the receiver is identical with that of the signal transmitted by the transmitter, the signal actually received by the receiver is not a single signal but rather the summation of signals transmitted thereto by differing paths. While one or more shortest-distance paths interconnect the transmitter and the receiver, a multiplicity of other signal paths also interconnect the transmitter and the receiver. For instance, the signal transmitted by the transmitter may be reflected off of both man-made or natural objects prior to reception by the receiver and signals transmitted upon such paths are received by the receiver, delayed in time relative to signals transmitted upon the shortest-distance paths. Because of such multiplicity of transmission paths, an actual communication channel is oftentimes referred to as a multipath channel and the signal received by the receiver is, hence, a summation of the plurality of signals transmitted thereto along the multiplicity of transmission paths. Because signals transmitted along other than the shortest-distance transmission paths arrive at the receiver delayed in time relative to the signal transmitted along the shortest-distance transmission path late-arriving signals interfere with previously-arrived signals. When the signal transmitted by the transmitter comprises the modulated, digital code, such interference is referred to as intersymbol interference. When such intersymbol interference is significant, the signal actually transmitted by the transmitter cannot be recreated by the receiver.
Receivers have been constructed which have two or more spaced-apart antennas for receiving signals transmitted thereto. The signals received at one or the other of the two or more spaced-apart antennas is utilized by circuitry of the receiver to recreate the signal actually transmitted by the transmitter. The antennas are positioned in relative orientations (such as, in a two-antenna configuration, in a mutually-orthogonal orientation) such that when a signal received at one of the antennas includes significant interference or is weak, a signal received at another of the antennas includes, typically, a lesser amount of interference or is of a greater strength. When two or more antennas are configured in such manner, the antennas are referred to as being in diversity (or, diversity antennas), and a receiver including such antennas configured in diversity are referred to as diversity receivers. And, transceivers including such antennas are referred to as diversity transceivers.
When the two or more antennas configured in diversity are spaced-apart from one another by more than one quarter wavelengths of the frequencies of the signals transmitted to the receiver, such antenna configurations work fairly well to overcome the effects of intersymbol interference. However, when the antennas positioned in diversity are positioned in close proximity to one another, coupling between the closely-positioned antennas degrades the antenna performance and, hence, similarly degrades the operability of a receiver incorporating such antennas to recreate a signal actually transmitted thereto by a transmitter.
As radiotelephone constructions utilized in cellular communication systems are constructed to be of increasingly smaller dimensions, antennas positioned in diversity and forming portions of such radiotelephones cannot be positioned apart from one another by the distances greater than the one quarter wavelength distances.
What is needed, therefore, is a diversity antenna structure which permits antennas configured in diversity to be positioned in close proximity to one another without causing significant intercoupling between the antennas.